Displays, such as, active matrix liquid crystal displays (AMLCDs), often utilize a back light source that is capable of producing wide dynamic luminance ranges. The back light source generally includes a resonant inverter with a transformer and a drive circuit and a fluorescent tube or lamp.
The drive circuit generates a low voltage, high frequency alternating current (AC) signal which is received by a primary winding of the transformer. The transformer steps up the low voltage, high frequency AC signal to a high voltage, high frequency AC signal that is provided by a secondary winding of the transformer to the fluorescent lamp. The high voltage, high frequency AC signal is typically provided to a high voltage output to which the lamp is attached. The fluorescent lamp emits light in response to the high voltage, high frequency AC signal from the transformer.
If the lamp is defective (e.g., broken) or disconnected from the socket, the voltage at the high voltage output can reach excessive levels. These extremely high voltages can cause catastrophic transformer insulation failure. Transformer insulation failure can result in a malfunction of the display system as well as create smoke and arcing which can be a considerable nuisance in an aircraft cockpit.
Replacing a display system such as in an aircraft application is particularly difficult due to the top level assembly associated with back light sources. Additionally, labor costs associated with replacing aircraft equipment are expensive. Therefore, there is a need to monitor the voltage level at the high voltage output and to prevent transformer insulation failures.
Heretofore, back light sources or display systems have determined whether an over-voltage condition exists at the high voltage output by directly sensing the voltage at the high voltage output. However, sensing the high voltage, high frequency AC signal at the high voltage output requires high voltage components which are expensive and must be isolated from lower voltage logic circuitry. Therefore, conventional over-voltage circuitry is expensive.
Thus, there is a need for a less expensive over-voltage protection circuit for a display system. Further still, there is a need for a back light source in a display system that includes over-voltage protection.